Nacreous mica pigment compositions

ABSTRACT

Nacreous pigments exhibiting intense interference colors, reduced milkiness and substantial luster are prepared by depositing a thin layer of titanium dioxide on mica flakes, treating the resulting titanium dioxide-coated mica pigment with a soluble silicate in an aqueous slurry at a pH from 7 to 11.5, and calcining the resulting silicate-treated pigment; and the resulting pigment.

[151 3,650,790 [451 Mar. 21, W72

NACREOUS MICA PKGMENT COMPOSITIONS Inventors: Edward F. Klenke, LibertyCorner, N.J.;

George L. Lewis, Wilmington, Del.

Assignee: E. I. du Pont de Nemours and Company,

, Wilmington, Del.

Filed: Feb. 13, 1970 App1.No.: 11,353

U.S. C1 ..106/291, 106/308 B Int. Cl. .C08h 17/04, C09c 1/28 Field ofSearch 106/308 B, 291, 309

[56] References Cited UNITED STATES PATENTS 3,087,828 4/1963 Linton106/291 3,532,528 10/1970 Bradshaw et a1 ..106/29l Primary ExaminerJamesE. Poer AttorneyFrank R. Ortolani [5 7] ABSTRACT 6 Claims, N0 DrawingsBACKGROUND OF THE INVENTION This invention relates to nacreous pigmentcompositions comprising silicate treated particulate metal oxide on micasubstrate.

Nacreous pigments are known and presently widely used. Such nacreouspigments are described in U.S. Pat. Nos. 3,087,828 and 3,087,829 toLinton, and are composed of a translucent layer of titanium dioxide orzirconium dioxide particles deposited on micaceous flake.

The color and other optical properties of nacreous pigment vary with thedepth of the metal oxide layer. When this oxide layer is about to 100millimicrons in thickness, the pigment exhibits a silver color. As thethickness is increased up to the maximum of about 250 millimicrons, thepigment exhibits various colors of the spectrum recurring in cyclicalsuccession, the cycles being referred to as first, second or third ordercolors, due to the optical phenomenon known as interference.Interference is the result of the reflectance of light from the top andbottom surfaces of a film resulting either in a reduction in theintensity of certain wave lengths of the incident light, which is knownas destructive interference, or reinforcement of the intensity of theincident light, which is known as constructive interference. When thethickness of the film is such that a ray reflected from the top surfaceofa film is out of phase with a ray that has passed through the film andhas been reflected from the bottom surface, there is destructiveinterference. Further discussion of the optical principles which explaininterference colors may be found in such textbooks as PrinciplesofOptics, by Born and Wolf, 2nd Ed. 1964, Chapter 7, (Macmillan Co.).

SUMMARY OF THE INVENTION The improved nacreous pigment of this inventionis mica flake substrate coated with hydrous metal oxide particles, whichis thereafter treated with a soluble silicate in an aqueous slurry, andis then calcined. It has been discovered that the transparency, luster,interference color, and optical properties of the translucent hydrousmetal oxide layer coated on mica flake pigment is improved by applyingsilicate thereto before calcining the pigment. In general a solublesilicate in aqueous solution is added to a slurry of the oxide-coatedmica flakes with adjustment of the pH until the pH ranges from slightlyalkaline to 11.5. The slurry is maintained in the pH range of? to 11.5for a short period of time before it is filtered, and the recoveredpigment is then calcined in the standard manner. Calcining dehydratesthe metal oxide and the silicate, which are hydrous as applied to themica substrate.

The resulting silicate-treated coated mica of this invention exhibitsmore intense color, greater luster and transparency in formulationsemploying pigments, thereby reducing the quantity of such pigmentrequired. This marked improvement in pigment quality and utility alsoappears as an absence ofmilkiness of white overtone.

More particularly, in practice of the invention, raw or uncalcined metaloxide-coated mica is prepared according to well known procedures 'suchas those in U.S. Pat. No. 3,087,828 to Linton, which is herebyincorporated herein in its entirety. As shown in Example 1 of thatpatent, mica flake is slurried in an aqueous solution of titanylsulfate. The agitated slurry is heated to the boiling point, and boilingis continued for a period of time sufficient to hydrolyze and depositthe desired amount of titanium dioxide on the mica flake. The titaniumdioxide-coated mica flake is recovered by filtering and washing withwater. For purposes of the present invention, the isolated and washedoxide-coated mica is then reslurried in water for silicate treatment.

The reslurried titanium oxide-coated mica is agitated while a solutionofa soluble silicate is added to the slurry. Sufficient silicate isadded to the slurry to adjust the pH to above at least 7.0 andpreferably to about 9.0 for a complete and reproducible treatment. Highsilicate concentrations and long periods of agitation increase the rateof treatment but are not necessary for satisfactory coating of thepigment. After addition of the silicate and agitation for a short periodof time, the silicatetreated, oxide-coated mica is recovered, as byfiltration and then is calcined in the conventional manner, i.e., atabout 800 to l,000 C., without intermediate washing.

Calcining the silicate-treated metal oxide-coated mica flake convertsthe hydrous metal oxide and silicate to the corresponding oxides anddehydrates the layers. This conversion must be relatively complete andreproducible to assure the intended final pigment color and quality.Calcination results in a decrease in the thickness of the coatings and ashift in inter ference color corresponding to the new or calcinedoxidecoating thickness with some adjustment for the silicate treatment.Experience shows that silicate in small amounts up to 10 percentinitially applied to the oxide-coated mica produces the expectedinterference color shift; as more silicate is used in the slurry, thecolor shift becomes constant indicating an end-point in treating themetal oxide layer. Above this point additional silicate does not appearto cause additional interference color shift.

The silicate employed in this discovery is thought to neutralize acid inthe metal oxide layer leaving in place of the acid radicals silicatewhich bonds, fills and smoothes the metal oxide layer. Any water solublesilicate which does not adversely affect the other components may beused, but sodium silicate is preferred for its obvious advantages ofavailability, economy and purity. As noted, sufficient of the silicateis used to effect treatment in the alkaline pH range. The amount ofsilicate as silica left on the pigment after calcination can range fromabout 3.5 to 25 percent and from 16 to 22 percent is the preferredamount.

Satisfactory mica substrate for use with this invention is commerciallyavailable. Generally the mica flakes can range from those passingthrough 140 mesh screen (U.S. Standard Sieve Series) down to a lowerlimit of about five microns. A preferred grade of mica is a water groundwhite Muscovite mica with flakes ranging in size from approximately 5 tomicrons in the largest dimension, with the major portion in the 5 to 50micron range. A product of this type is available under the nameAlsibronz mica from Franklin Mineral Products Co., Wilmington, Mass. Inaddition, biotite, related vermiculite, and various synthetic micas inthe described particle size range, and especially those which resemblewhite Muscovite mica, can also be used.

The invention is further illustrated in the following examples. Allparts and percentages are by weight unless otherwise indicated.

EXAMPLE 1 Titanium dioxide-coated mica is prepared generally accordingto known procedures, such as that described in U.S. Pat. No. 3,087,828to Linton. In this example, a water ground Muskovite mica, iswet-classified to remove mica flakes less than 5 microns in size. Themica used in this example is available under the name Alsibronz. A 37.4g. portion of the wetclassified Alsibronz mica is slurried in 100 cc. ofwater at 70 C. An aqueous titanyl sulfate solution weighingapproximately 100 grams and containing the equivalent of 14.5 grams ofTiO is added to the slurry at a unifonn rate in a period of about fiveminutes. A 50 g. portion of 50 percent sulfuric acid is added. Theslurry is stirred for one hour and heated to boiling in a period ofone-half hour. Boiling is continued until the desired amount of titaniumdioxide for the desired color is deposited on the mica as indicated by avisual in process water spot test.

The in process water spot test consists of depositing on a blackbackground a drop from the boiling slurry containing the mica beingcoated, diluting this drop with an equal amount of water, and observingthe color of the coated mica in the drop. Due to changes in the coatedmica upon subsequent treatment, color of the water spot test is not thesame as the final coated mica color. The final TiO coated mica color iscorrelated with the in process" water spot test and the subsequenttreatment methods so that the desired final color is produced. For thisexample, boiling is stopped when the water spot test shows a red-goldcolor for the coated mica. The slurry is filtered to recover thetitanium dioxide-coated mica and the filter cake is washed sulfate free.

A 3 gm. portion of coated mica prepared as just described is slurried in30 cc. of water at room temperature. Sodium silicate solution is addeduntil the pH of the resulting mixture is 9.5 to 10.0. The sodiumsilicate solution contains 28.4 percent silica (W. W. Grade, Du Pont) inwater. The slurry is stirred for 15 minutes at room temperature and isthen filtered to recover the silicate-treated titanium dioxide-coatedmica. The silicate-treated titanium dioxide-coated mica is finished bycalcining between 930 and 950 C. for one-half hour. The silica contentis about 16 to 22 percent ofthe pigment.

A second portion of the titanium dioxide-coated mica prepared accordingto the procedure described but with no silicate treatment is finished inthe standard manner as a control. This is calcined at 930 to 950 C. forone-half hour and is used as a standard for comparison with thesilicate-treated portion.

These two pigments are compared side by side in a formulation consistingof 0.75 g. of the pigment in 10 g. of acrylic lacquer. A drawdown ismade on a black and white comparison card so that the colors of the twosamples can be compared. The interference color of each sample appearsclearly on the black half of the sample card, so that the difference inthe two samples is readily apparent.

The final color of these samples is predominantly gold. Thesilicate-treated sample clearly appears to be more golden with less redovertone (caused by slight interference color shift), to be less milkyor to have little or no hazy overtone, and to have a more intense goldencolor. The samples are observed and rated on an arbitrary scale ofO to99, with to standard. The results for two samples prepared as indicatedare given as follows:

Using the same procedure as Example 1, a second set of samples isprepared with slight variations: A 25 g. portion of wet-classifiedAlsibronz" is slurried in 200 cc. of water at 70 C. A 100 gm. portion ofaqueous titanyl sulfate solution (14.5% TiO is added over a -minuteperiod. A 40 cc. portion of 50 percent sulfuric acid is added and theslurry is stirred for l hour. The slurry is heated to boiling over a &-hour period and boiled until the "in process" water spot test shows ablue color (which correlates with a final red color). Boiling is stoppedand the slurry is filtered. The titanium dioxide-coated mica is washedsulfate free for subsequent treatment.

One portion of the coated mica is slurried in 100 cc. of water at roomtemperature. Sodium silicate solution as in Example 1 is added to adjustthe pH to 9.510.0. The slurry is stirred for minutes, filtered, anddried. The silicate-treated titanium dioxide coated mica is calcined atabout 930 to 950 C. for one-half hour. For comparison, a second portionof the coated mica but with no silicate treatment is calcined at thesame conditions.

As in Example I, drawdowns of each are made and compared, the followingdata being obtained:

Following the same general procedure as Example 1, but using 16 grams ofA1sibron2" as received from the supplier, hydrolysis of the aqueoustitanyl sulfate is carried out until the in process color test shows asecond-order purple color (which correlates with a final green color).The coated substrate is recovered and subsequently finished withsilicate treatment exactly as in Example 1. Drawdowns are again made andcompared and the same pattern of distinct and general superiority isfound in the silicate treated pigment.

From the foregoing examples and disclosure it is apparent that thisdiscovery provides an effective way to prepare pigment of pearl throughsecond order red of improved intensity of color and lacking inmilkiness. This is achieved by treat ing metal oxide coated mica with asoluble silicate, such as sodium or other alkali metal silicate, asdetailed. The quantity of metal oxide applied for any given color is afactor known to the art, but generally ranges from about 20 percent (forpearl) to about 50 percent (for copper) based on the pigment weight.While the examples show the use of titanium dioxide, it is consideredthat zirconium oxide can as well be used while achieving the advantagesindicated.

What is claimed is:

l. A process for making a nacreous pigment composition composed of micasubstrate in flake form coated with hydrous metal oxide particles as atranslucent layer of about 20 to about 250 millimicrons in thickness,and wherein said particles have a size of less than about 0.1 micron,comprising suspending mica flake substrate in an aqueous slurry,depositing hydrous metal oxide particles as a translucent coating onsaid mica flake, treating the resulting flake in an aqueous slurry witha soluble silicate in an amount such to provide a pH range from about 7to 11.5, recovering the silicate treated flakes and then calcining them.

2. The process of claim 1 in which the hydrous metal oxide is titaniumdioxide.

3. The process of claim 2 in which the soluble silicate is sodiumsilicate.

4. The process of claim 3 in which the least 9.0.

5. A nacreous pigment composition consisting essentially of coated micaflake, said coating being a translucent layer having a thickness ofabout 20 to 250 millimicrons of metal oxide particles substantially allof which are less than 0.1 micron, said metal oxide coated mica flakehaving been treated with a soluble silicate so that silica remainingassociated with the layer of metal oxide particles is equal to fromabout 3.5 to 25 percent ofthe total pigment weight.

6. A pigment according to claim 5 in which said silica content is about16 to 22 percent and said metal oxide is titanium dioxide.

pH of the slurry is at

2. The process of claim 1 in which the hydrOus metal oxide is titaniumdioxide.
 3. The process of claim 2 in which the soluble silicate issodium silicate.
 4. The process of claim 3 in which the pH of the slurryis at least 9.0.
 5. A nacreous pigment composition consistingessentially of coated mica flake, said coating being a translucent layerhaving a thickness of about 20 to 250 millimicrons of metal oxideparticles substantially all of which are less than 0.1 micron, saidmetal oxide coated mica flake having been treated with a solublesilicate so that silica remaining associated with the layer of metaloxide particles is equal to from about 3.5 to 25 percent of the totalpigment weight.
 6. A pigment according to claim 5 in which said silicacontent is about 16 to 22 percent and said metal oxide is titaniumdioxide.